.beta.-Isophorone is of great interest economically because it is an important synthetic structural unit for the preparation of carotinoids, vitamins and pharmaceutical products. .beta.-Isophorone in particular is necessary as a precursor for ketoisophorone (=2,6,6-trimethylcyclohex-2-en-1 ,4-dione) and trimethylhydroquinone and hence for the preparation of vitamin E. In addition, it plays a crucial part in syntheses of perfumes and natural substances such as astaxanthin and abscisic acid and derivatives.
Isophorone is prepared by trimerization of acetone, with condensation of the C.sub.3 structural units. .alpha.-Isophorone is the main isomer formed because, unlike the .beta.-isomer, it possesses a double bond conjugated to the keto function. For this reason, the thermodynamic equilibrium lies towards the .alpha.-isophorone; the .beta.-concentration is only about 1-2%, depending upon the temperature, and the equilibrium is established very slowly.
Although there are in principle two different methods of obtaining ketoisophorone, namely, the direct oxidation of .alpha.-isophorone .fwdarw. ketoisophorone, and the detour via the isomerization of .alpha.-isophorone .fwdarw. .beta.-isophorone in an initial step and subsequent oxidation of .beta.-isophorone .fwdarw. ketoisophorone, the latter process is clearly more advantageous. Scheme 1 illustrates these observations on the synthesis of ketoisophorone: ##STR1##
Over the years numerous processes for the isomerization of .alpha.-IP have been described, which nevertheless have considerable disadvantages. Aspects such as high consumption of chemicals, poor space-time yield and problems during the working-up have hitherto prevented a practical transfer of the process to a larger scale.
In the processes for preparing .beta.-IP from .alpha.-IP, a distinction can be made between gas-phase reactions and liquid-phase reactions.
Four parallel reactions of .alpha.-isophorone in the gas phase are possible in principle. These reactions compete with one another and succeed to a varying degree depending upon the selected temperature range and the surface condition of the catalyst employed.
In the gas phase, isophorone can react on contact in the following ways:
a) isomerization to .beta.-isophorone PA1 b) reduction to trimethylcyclohexadienes (the hydrogen necessary for this is supplied through the decomposition of IP, which is accompanied by coking phenomena) PA1 c) .beta.-elimination of methane to 3,5-xylenol PA1 d) formation of mesitylene. PA1 D1=A. Heymes et al., Recherches 1971, 18, 104 PA1 D2=FR-A-1 446 246 PA1 D3=DE-OS-24 57 157 PA1 D4=U.S. Pat. No. 4,005,145 PA1 D5=EP-A-0 312 735 PA1 D6=JP87-33019 eq. to HEI-1-175954 v. 12.07.1989
The catalyzed reactions of .alpha.-IP in the gas phase on a heterogeneous contact are shown in the following Scheme 2: ##STR2##
EP 0 488 045 B1 discloses an isomerization process in the gas phase (300-450.degree. C.) above a heterogeneous catalyst. The catalysts used are oxides and mixed oxides of Mg (group IIa), AI (IIIa), Si (IVa) and Ni (VIII), which are active per se or are applied to a y-aluminium oxide support (specific surface 1-50 m.sup.2 /g). 1-10 kg .beta.-IP is used per liter of catalyst; the concentration of the solution obtained as intermediate is 9% .beta.-IP at most, depending on the catalyst loading; the end concentration after distillation under vacuum is 97% .beta.-IP. NiO is granulated using 1% Luviskol K90 (- polyvinylpyrrolidone). Under optimal conditions, a catalyst performance of 0.33 liter .beta.-IP/h/liter.sub.cat is achieved when this procedure is employed. Based on the volume of educt used, the space-time yield Y.sub.R-Z =0.09 liter.sub..beta.-IP /h/liter.sub.solution (Ex. 1).
Moreover, the withdrawal rate is low, which renders the process less attractive on the industrial scale.
In L. F. Korzhova, Y. V. Churkin and K. M. Vaisberg, Petrol. Chem Vol. 31, 1991, 678 the reaction of .alpha.-IP at 300-800.degree. C. in the presence of heterogeneous catalysts is described. The catalytic systems considered are .gamma.-aluminum oxide, magnesium oxide and quartz. The range of products is examined in relation to temperature and catalyst. The formation of .beta.-IP, trimethylcyclohexadiene, 3,5-xylenol and of mesitylene are compared with one another (see Scheme 2: routes a., b., c.; d.). Thus the thermal decomposition of .alpha.-IP at above 550.degree. C. on a less developed catalytic surface (quartz) yields a mixture having the composition c&gt;&gt;a&gt;&gt;d and b=0. The reaction on the MgO contact at 400.degree. C. shows a similar range of products at a significantly lower temperature, namely c&gt;&gt;a&gt;d&gt;b. In the presence of an aluminium oxide catalyst having a marked basic-acidic surface structure, the reaction takes place at 300.degree. C. with a clear preference for the cyclohexadiene products, namely b&gt;&gt;c&gt;d.
Altogether, it can be assumed that a catalytic gas phase isomerization as several quite definite disadvantages: in general, it can be said that these processes are disadvantageous because either the formation of the product is accompanied by a considerable accumulation of secondary products, or the space-time yield (absolute .beta.-IP-formation/h/kg.sub.cat) is too low.
There are also a number of publications which deal with the isomerization in the liquid phase. The closest prior art is represented by the following documents:
D1 discloses the isomerization of (.alpha.-IP to .beta.-IP using stoichiometric quantities of MeMgX (X=halogen-), a Grignard compound. In the presence of catalytic quantities of FeCl.sub.3, 73% .beta.-IP is obtained, with release of methane. Mechanistic concepts assume that the Grignard compound reacts as a base and does not function as the carrier of a carbanion. Excess Mg leads to the formation of mixtures of dimers, which are the result of a reducing metallic dimerization. However, the reaction of .alpha.-isophorone with molar quantities of methylmagnesium iodide in the presence of catalytic quantities of FeCl.sub.3, the subsequent hydrolysis and the working-up by distillation is a complicated procedure as well as being expensive as regards chemicals.
D2 relates to the isomerization of .alpha.-IP to .beta.-IP in the presence of catalytic quantities of p-toluenesulfonic acid and aromatic sulfonic acids generally, in particular anilinesulfonic acid. The quantity of the catalyst used is 0.1-0.2%, based on the .alpha.-IP used. However, a lower degree of conversion and a greater accumulation of secondary products prevents an industrial application of the process in D2.
According to D3, .beta.-IP is prepared by boiling .alpha.-IP for several hours in triethanolamine, fractionating and then washing the distillate with tartaric acid and common salt solution. Again, the consumption of chemicals and the labor expended in the process are considerable.
In D4, acids having a pK=2-5 and a boiling point higher than that of .beta.-IP (bp .beta.-IP=186.degree. C./760 mm Hg) are used as catalyst. The patent claim explicitly protects the following compounds in the liquid phase:
aliphatic and aromatic amino acids, adipic acid, p-methyl-benzoic acid, 4-nitro-m-methylbenzoic acid, 4-hydroxy-benzoic acid, 3,4,5-trimethoxybenzoic acid, vanillic acid, 4-trifluormethylbenzoic acid, 3-hydroxy-4-nitrobenzoic acid and cyclohexanecarboxylic acid and derivatives.
The quantity of catalyst used is 0.1-20 mol.%. The yield of .beta.-IP (based on .alpha.-IP used) and therefore the selectivity is 74.5%. Under the given conditions this corresponds, converted to the quantity of catalyst used and time, to a yield Y=0.218 liters .beta.-IP per kilogram of catalyst per hour.
The homogeneous catalytic isomerization of .alpha.-IP to .beta.-IP accompanied by little dissociated acid is an improvement as regards the consumption of chemicals, with .beta.-IP being continuously removed from the equilibrium. With a withdrawal rate as low as 11 ml/h .beta.-IP at an input of about 0.5 kg .alpha.-IP, the space-time yield and the formation of .beta.-IP has a value Y=0.24 kg .beta.-IP/kg.sub.cat /h, which is too low for industrial application.
A similar principle is followed in D5. Acetylacetonates of transition metals are used as catalysts for shifting the .pi. bonds. Al(acac).sub.3 also exhibits catalytic activity. The catalyst is used in a quantity of 0.01-10 wt %. The catalysts patented are metal catalysts of the groups IVb (Ti/Zr/Hf), Vb (V/Nb/Ta), VIb (Cr, Mo, W), VIIb (Mn/ Tc/Re), the whole of group VIII and aluminum. The primary distillate obtained has a .beta.-IP content of 94%, a further Vigreux distillation concentrates the .beta.-IP content to 99%. Based on the quantity of catalyst used and the time, this result corresponds to a yield Y=9.4 liters .beta.-IP per kilogram of catalyst per hour. Based on the educt solution used, this corresponds to a yield Y.sub.R-Z =0.0376 liter.sub..beta.-IP /h/liter.sub.solution.
Apart from the fact that the space-time yield is low and the accumulation of secondary products is considerable, the catalyst and distillation residue are not easily separated in the homogeneous catalyst system used. It is therefore essential to discard material from time to time, as otherwise the temperature in the bottom of the distillation column would rise excessively. Thus a "monitoring" of the temperature is necessary.
According to D6, the isomerization in the liquid phase is carried out at temperatures of around 200.degree. C. The catalyst used is silica gel with or without addition of alkyl-substituted imidazolines corresponding to the following formula. ##STR3##
Typical experimental conditions: 300 g .alpha.-IP and 25.7 g SiO.sub.2 are distilled for 52 h in the presence of special steel; this results in the recovery of 230 g .beta.-IP (=76.6% yield) with 99.9% purity. Based on the quantity of catalyst used and the time, this result corresponds to a yield Y=0.174 liters .beta.-IP per liter of catalyst per hour.
But the preparation of the organic bases is costly and the space-time yield of the process is low; with a typical value of Y=0.174 liters .beta.-IP/liter cat/h, this process is not transferable to an industrial scale either Based on the volume of educt solution used, the yield Y.sub.R-Z =0.0149 liter.sub..beta.-IP /h/liter.sub.solution .
The procedure described is moreover unfavorable and the absolute formation of .beta.-IP is low. A particular disadvantage is the batchwise operation and the carrying out of the isomerization and purifying distillation of the .beta.-IP in a single step. As a result of the high reaction temperature in the distillation apparatus, there is demonstrably a considerable reverse isomerization of .beta.-IP to .alpha.-IP.